Many digital still image and video cameras employ a passive type auto focus system that measures the contrast of image content. Generally described, when comparing a sharp image with a blurred image, rendered on the same scene, the sharp image contains more information from high spatial frequencies. There are more transitions between dark and bright areas and the difference between the dark and bright areas is greater. Assessing the contrast of image content between images can give a relative measure of sharpness. A conventional contrast-based auto focus system includes a camera lens, a focusing means for changing the position of the lens, an imaging sensor and a digital signal processor (DSP). The lens is configured to move to different discrete focusing positions, each corresponding to a particular subject-to-camera distance. In operation, the system moves the lens to a number of different positions during the auto focusing process. An image is captured at each position and a relative sharpness value can be determined for each image. The system then moves the lens to the position that produced the image having the greatest sharpness value.
A typical mobile phone digital camera uses the image contrast technique for auto-focusing. The mobile phone camera may cover a focusing distance of from about 10 cm to infinity by using a resolution accomplished by between 10-20 focusing steps. Such a mobile phone camera may have a 2 million pixel sensor and a fixed focal length lens with an aperture f1:2.8. This type of camera may produce about 15-30 images every second in daylight or bright artificial light. It is believed that the auto focus actions for an image using these devices is relatively slow can take about 1-2 seconds to perform.
Sub-sampling can be used to shorten this time. In the sub-sampling method, the lens does not move to every discrete location during the search process. The sharpness values can be interpolated to produce an estimation of sharpness for the positions between those actually sampled.
Unfortunately, while sub-sampling may shorten the auto focus time, a degradation in precision may be introduced. In addition, the time it takes to focus is still relatively long, typically about 1 second. This may make it difficult to achieve a good tracking auto focus for video recording using the sub-sampling technique. For example, when the main subject is moving closer or farther away from the camera, the system can indicate that the sharpness has changed. However, it is difficult to determine how much the subject has moved and/or in which direction. Some relatively sophisticated and precise (relatively expensive) video systems may address this problem by “hunting” the lens back and forth in very small steps.
There remains a need to provide alternative economic auto focus systems for digital cameras with still and/or video recording capability.